Classic Car Engine Oil: Grades, ZDDP and Running-In
Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent
The right classic car engine oil is the grade your engine was designed for, with enough anti-wear additive to protect its camshaft. For many older engines that means a classic oil with ZDDP, often a 20W-50 multigrade or a monograde for very early cars. A freshly rebuilt engine needs a running-in oil first.
Every engine is different, so always start with the handbook or workshop manual for your car.
Why classic engines need different oil
Older engines were built in a different way to modern ones. Nigel Bottom of the Witham Group, writing in Professional Motor Mechanic, says they were designed with “looser tolerances, flat-tappet camshafts and relatively low operating temperatures”.
He adds that flat-tappet cams, cork seals and rope rear main seals all place “unique demands on lubricant chemistry”. A modern oil is made for a modern engine with tight clearances, a catalytic converter and long service intervals. That is not what a 1960s engine needs.
If you drive a modern car, our guide on which engine oil your car needs is the better place to start.
Monograde vs multigrade
A monograde oil has one viscosity rating, such as SAE 30 or SAE 40. A multigrade has two, such as 20W-50. The first number is about how the oil behaves when cold, the second about how it behaves when hot.
According to Bottom, early monograde oils such as SAE 30 and SAE 40 “had limited additive packages” and relied on frequent oil changes. Multigrades such as 10W-40 then “improved cold-start protection”.
He also notes that “many classics were designed around 20W-50 oils, and that thicker film can be vital for worn clearances”. So a thin modern oil is usually the wrong choice for an older engine.
Which do you need? That depends on the age of the engine and what the maker specified:
- Pre-war and very early cars, especially those with only an oil strainer, may call for a non-detergent monograde.
- Many post-war classics were designed for a 20W-50 multigrade.
- A rebuilt engine may have been built to a different specification, so ask whoever rebuilt it.
Classic car needs some care?
Servicing, recommissioning and mechanical restoration for older cars across Kent.
WhatsApp for a free quote Call 07563 350335
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
What ZDDP is and why it matters
ZDDP stands for zinc dialkyldithiophosphate. It is an anti-wear additive.
In an article from Castrol’s Technical Centre, reproduced by the MG Car Club V8 Register, Castrol says ZDDP “provides a high level of anti-wear protection”. It explains that ZDDP levels were reduced in modern oils because “its phosphorus content is harmful to catalytic converters and other emission equipment”.
That matters for older engines with flat tappets. Bottom says “many classic engines rely on higher ZDDP levels to protect flat-tappet camshafts”, and that modern low-zinc oils “can accelerate wear in older valvetrain designs”. Castrol warns that without enough anti-wear additive, the oil film can break down and cause metal-to-metal contact.
In practice, this is why classic oils exist. They keep ZDDP at levels suited to older engines. If your engine has flat tappets, a classic oil with ZDDP is the safe choice.
Detergent and non-detergent oil
Modern oils contain detergents that keep dirt suspended in the oil so the filter can catch it. Bottom says detergents “were introduced to keep engines cleaner”.
But very early engines often had no full-flow oil filter, just a strainer. For these, non-detergent oils are still made. Millers Oils, for example, describes its Vintage Millerol range as “non-detergent monograde mineral” oils, “specifically engineered for vintage cars with a strainer or basic filtration systems”.
There is also a risk with old, unrestored engines. Bottom warns that “highly detergent modern oils can dislodge decades of harmless sludge”, and that “controlled-cleanliness formulations are often safer for original units”. He also notes that modern synthetic oils “can be more aggressive on aged seals and gaskets”.
Do not switch an old engine from one type of oil to another without knowing why. If you are not sure what has been used before, ask a mechanic to look at the engine first.
Running-in oil after a rebuild
A rebuilt engine has new surfaces that need to bed in. Bottom says “fresh rebuilds need running-in oils with appropriate additive levels”.
As an example, Millers Oils describes its Classic Running In Oil as an SAE 30 mineral oil “designed for the initial running in of re-built engines”. It says the oil is suitable for 500 miles, after which it should be drained and replaced with a suitable classic engine oil. It also warns: “Do not use high performance oils before the bedding-in process is complete.”
Castrol adds that care should always be taken to follow the manufacturer’s recommendations when running in new parts such as camshafts. Always follow the instructions from your engine builder and the oil maker. Different engines and products can have different procedures.
Our post on classic car engine rebuilds in Kent explains what a rebuild involves.
How often to change classic oil
Classic oil change intervals are usually much shorter than modern ones, and they vary from car to car. Check your handbook or service book.
Time matters as well as miles. Bottom says “ageing oil causes corrosion even when the vehicle is rarely driven”. So a car that only covers a few hundred miles a year still needs fresh oil regularly. Many owners change it before laying the car up for winter.
If you own an MG, our MGB maintenance guide covers that car in more detail.
How we can help
We are a mobile mechanic based in Aylesham, offering a classic and vintage car mechanic service in Kent at your home or garage. We cover Canterbury, Dover, Deal, Sandwich, Folkestone and the villages in between.
We do oil and filter changes, full classic servicing, recommissioning and air-cooled VW engines. Classic engine rebuilds are done in a Kent workshop. Labour is £30-£50 per hour depending on the job and its difficulty. Parts and oil are extra, and we agree the price on WhatsApp before any work starts.
Frequently asked questions
Can I use modern oil in my classic car?
It depends on the engine. Many classics with flat tappets need a higher ZDDP level than modern oils provide. Check the handbook and use a classic oil if in doubt.
Do I need non-detergent oil?
Usually only for very early engines with a strainer or basic filtration, where the maker calls for it. Check the handbook or ask a mechanic.
How long should I use running-in oil?
Follow the oil maker and your engine builder. Millers says 500 miles for its Classic Running In Oil, then change to a classic engine oil.
Should I change the oil if the car hardly moves?
Yes. Old oil can cause corrosion even in a car that is rarely driven.
Need help? Send a WhatsApp to 07563 350335 with your car’s make and model and we’ll come to you.
Classic car needs some care?
Message us on WhatsApp with your reg and postcode for a price before any work starts.
WhatsApp for a free quote Call 07563 350335
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
